"The Gods Themselves" by Isaac Asimov

Asimov doesn't usually write about aliens.  Also, Asimov doesn't usually write about sex.  Well, this book, Asimov's favorite and Hugo and Nebula winner for 1972 is mostly about aliens with quite a bit of sex thrown into.  It is mostly the aliens that have sex, though.  Humans have a go at it, in the last line of the novel, but we're left out of the details. 

The novel consist of three parts; part 1 happening on Earth, part 2 in the ParaWorld, and part 3 on the Moon.  In Part 1 a mediocre scientist, Hallan, discovers that a certain material in his lab is getting exchanged with a radioactive isotope that cannot exist in our universe, with its strong and weak nuclear forces as they are.  He soon discovers that aliens from a parallel universe (ParaWorld) are exchanging the materials, each of which becomes radioactive in the other universe and emits free energy until it stabilizes.  This is used by Earth people to construct "Electron Pump" stations which provide free, clean, unlimited energy, obfuscating all other sources of energy.   Hallan becomes the most famous scientist on Earth and grows into vindictive, vicious fellow who destroys anyone who questions his authority, as is the case with a scientist historian who claims that it was actually the ParaMen who invented the pump, and Hallan has only been their puppet.  The historian also claims that the pump can destroy our universe (or at least our part of the Galaxy) because it is subtly changing the natural laws. 

In the second part we learn about the ParaMen who exist in a universe where matter is not as tightly woven together and the creatures on the ParaWorld can 'meld' with each other, which is how they propagate, and some can also 'meld' into solid rock, being able to turn themselves into almost a mist.  The ParaMen have three sexes, left, right and middle, and a child can only be produced if all three sexes meld.  The ParaMen are worried because the sources of energy in their universe are disapearing, since all their suns are getting smaller, which is why they invented the "Positron Pump", but some of them have actually discovered the dangers of changing the natural laws, and the destruction that could follow.  One 'Middle' alien, Dua, tries to communicate the danger to the humans in the other universe, and even manages to spell FEER (fear) figuring out the language of the humans.

The third part is the weakest.  Happens on the moon, where Loonies and Immis live, and dislike the Earthies from the home planet.  Halans assistant from the original discovery moves to the moon to experiment on his theories that the Pump can destroy the universe, but doesn't have to, if he can construct a 'safety valve' of sorts.  The assistant meets a Loonie tour guide, who is actually an "Intuitionist" (coined by Asimov, intuitive scientist) who joins him in his research, despite being a member of an underground Loonie movement that seeks to remove the Moon from the Earth's orbit and leave the Solar system.   Eventually the experiment is successful, the universe is saved, Hallan's reputation destroyed, and the assistant and the tour guide are just about to have sex when the book ends.

Like most Asimov's books, it can be read as a scientific treatise and the science insights are stunning, however the characters and plot are paper-thin, and only serve to explain the amazing scientific conjectures.